LIGHT REGULATION OF FED-1 MESSENGER-RNA REQUIRES AN ELEMENT IN THE 5'-UNTRANSLATED REGION AND CORRELATES WITH DIFFERENTIAL POLYRIBOSOME ASSOCIATION

Light regulation of Fed-1 mRNA abundance in the leaves of green plants is primarily a post-transcriptional process. Previously, we have show n that the Fed-1 mRNA light response requires an open reading frame, i ndicating that the light regulation of the mRNA depends on its concurr ent translation. We now show that light-induced increases in Fed-1 mRN A abundance are associated with increases in polyribosome association that require both a functional AUG and a normal Fed-1 translational st art context. We also present evidence that light regulation of Fed-1 m RNA levels requires more than efficient translation per se, Substituti on of the efficiently translated tobacco mosaic virus Omega 5' untrans lated region resulted in a loss of Fed-1 light regulation. In addition , we identified a CATT repeat element located near the 5' terminus of the Fed-1 5' untranslated region that is essential for light regulatio n. We introduced two different mutations in the CATT repeat element, b ut only one of these substitutions blocked the normal light effect on polyribosome association, whereas both altered dark-induced Fed-1 mRNA disappearance, The element may thus be important for Fed-1 mRNA stabi lity rather than polyribosome loading. We propose a model in which Fed -1 mRNA is relatively stable when it is associated with polyribosomes in illuminated plants but in darkness is not polyribosome associated a nd is thus rapidly degraded by a process involving the CATT repeat ele ment.